Pseudopolis Yard was an area of the city of Ankh-Morpork that housed the headquarters of the Ankh-Morpork City Watch. It originally belonged to the Ramkins estate, but was gifted to the City Watch after their previous Watch house at Treacle Lane was destroyed by a dragon.
It is a reference to Scotland Yard, the headquarters of Metropolitan Police. Both places are headquarters for police forces named after places which are not situated in the city the building is located in.